The prior art discloses processes for preparing flexible foams with a low density and a low resilience from polyols having a high oxyethylene (EO) content and a polyisocyanate having a high 4,4′-MDI content (see e.g. EP 547765). Further, copending application EP 99105419.8 discloses a process for making a flexible polyurethane foam by reacting 4,4′-diphenylmethane diisocyanate, a polyol having a high oxyethylene content, and water.
However, the prior art does not provide a solution to the following problem. Namely, devices suitable for making flexible foams often are used for making flexible foams from different formulations containing different chemicals. It has now been observed that when foams are made from polyols that have a relatively high oxypropylene content (PO-polyol) before foams that are made from polyols having a relatively high oxyethylene content (EO-polyol), a severe deterioration of the later made foam occurs. It is possible that the sever deterioration is caused by a contamination, which may occur, for instance, because small amounts of PO-polyol are left in the polyol tank, the mixing device, and/or the feed from such tank to such device. Such contamination, of course, can be avoided by thoroughly cleaning or flushing the total device. This, however, is laborious, time-consuming, and environmentally less friendly because after flushing a lot of polluted waste material is obtained. Therefore, there is a need to find another solution to this problem.